Mobile wireless communication technology has gained such widespread acceptance throughout many densely populated areas that the bandwidths of the radio frequency (RF) spectrum that have been allocated for mobile phone systems are rapidly approaching their capacities using existing analog technologies. This rapid approach of the bands' capacity is being hastened by the increased applications of wireless communications for data transmission, such as facsimiles and Internet access, and video transmission. Still another application of wireless technology that is creating capacity concerns is the use of wireless systems for local, relatively small scale networks which lie within the coverage area of larger public networks. Examples of such small scale networks include cordless links between detection/control instrumentation in an assembly line and a central data collection computer, or a wireless telephone system within an office environment, i.e., a local area network (LAN). The larger networks would include commercial cellular phone service providers. Although frequently referred to as an "indoor cellular network" or "indoor system", the second, small scale system need not be literally indoors. Alternatively, a small scale or indoor cellular communication system may be referred to as a "pico-cell," with the outdoor cellular communication system being referred to as a "macro-cell."
Digital technologies are being implemented in an effort to increase the subscriber capacity of the allocated bandwidths as well as to provide higher quality signals for transmission. These digital techniques, which include TDMA (time division multiple access), digital FDMA (frequency division multiple access), and code division multiple access (CDMA), share the same available channel capacity by creating a number of subchannels, each by a different method. In digital FDMA systems, simultaneous transmissions are separated by dividing the available frequency band into disjoint sub-bands. In such a system, interference due to the co-existence of large scale and small scale wireless communications networks in a common area could be addressed by assigning specific frequencies within the available band to the different networks, sacrificing portions of both networks' capacities in order to enable the overlap. In a TDMA system, the signals are divided into disjoint time slots, with a common time reference providing means for coordinating the elements of the network. By further coordinating the time references for the overlaid large scale and small scale networks, different time delays can be used to distinguish between the signals within the respective networks to avoid interference. Other solutions to distinguishing the signals from two or more at least partially overlapping FDMA or TDMA networks are known and will be apparent to those skilled in the art.
In analog, TDMA and digital FDMA wireless communications technologies, the indoor communications system may operate in a different frequency band than outdoor systems, requiring frequency planning and exclusive allocation of portions of the frequency band to each system. Frequencies that are exclusively allocated to a certain system but remain unused due to low traffic in that system represent a waste of available radio resources.
In a CDMA system, multiple subscribers each use a channel identified by a unique digital code. CDMA provides several advantages over conventional digital FDMA or TDMA systems. A significant advantage is that frequency spectrum allocation planning for mobile and base stations within cells of a CDMA system is not required. Consequently, the capacity of a CDMA system is potentially greater than that of a TDMA or FDMA system. Additionally, since the energy of the transmitted signals in a CDMA system is spread over the wide band uplink or downlink frequency band, selective frequency fading does not effect the entire CDMA signal. Further, path diversity can be exploited in a CDMA system but must be compensated in other systems. (A comprehensive overview of CDMA system principles is provided in the book by Andrew J. Viterbi, entitled CDMA: Principles of Spread Spectrum Communication, Addison-Wesley Publishing, 1995.) The Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) IS-95-A standard, which has been accepted as one basis of commercial cellular networks in the USA, describes some of the important technical characteristics of a CDMA-based cellular radio telecommunication system. In addition to the IS-95-A standard, there is another common air interface standard for CDMA which is used for PCS networks. This standard, designated as ANSI J-STD-008, applies to CDMA communications within the 1850-1990 MHZ frequency band. Both cellular and PCS CDMA systems have the same basic signal structures, i.e., message formats, coding, and modulation, and, thus, are similarly appropriate for implementation of overlaid communications systems.
A CDMA base station uses one or more carrier frequencies by spreading a plurality of independent transmission streams into a frequency band that is associated to each carrier. This spreading is accomplished by coding the transmission stream with a distinct spreading code. These spreading codes are orthogonal to enable each receiver to cancel out all signals other than the one it wants to receive. Orthogonality requires synchronization on the bit level between different transmission streams in the transmitter. Decoding a CDMA transmission requires that the receiver be able to synchronize itself into the transmitted sequence at the chip level, where a chip represents the length of one bit in the transmission.
In spite of its advantages, the absence of frequency allocation in CDMA systems poses a unique problem in dealing with geographically overlaid, or partially overlaid, networks which are operating within the same frequency band. The IS-95 standard does not place restrictions on the size of cells of a cellular network, thus allowing the operation of overlaid radio communication systems such as the large scale and small scale networks described above. In a CDMA system with overlaid indoor and outdoor networks, intersystem interference results when transmissions in one system appear at the receivers of the other system, causing errors and signal corruption. This problem arises most often due to the relatively higher transmission power of the Outdoor Mobile Stations (OMSs) and Outdoor Base Stations (OBSs) as compared to the Indoor Base Stations (IBSs) and Indoor Mobile Stations (IMSs). An attempt to overcome this problem by boosting the relative power levels of the indoor system would likely only result in increased interference with the outdoor system by the indoor system's transmissions.
Another obstacle to implementation of overlaid CDMA communication systems is a significant near-far effect. The near-far effect is typically caused by an outdoor mobile station transmitting at a high power from a location near either an indoor base or mobile station. This outdoor transmission causes the receiver within the indoor system to be overwhelmed by the strong outdoor signal, thus being unable to detect the lower power indoor signals.
It would be desirable to provide a system and method which would allow the effective sharing of the same frequency bands between overlaid CDMA-based indoor and outdoor radio communication systems while simultaneously preventing interference between the two systems.